With the ongoing discussion of Earth structure under West Antarctica and how it relates to the extension and volcanism of the area, we explore the possibility of a hydrated or thermally perturbed mantle underneath the region. Using P-wave receiver functions, we focus on the Mantle Transition Zone (MTZ) and how its thickness fluctuates from the global average (240-260 km). Prior studies have explored the West Antarctic regions of Marie Byrd Land and the West Antarctic Rift, but we expand this to include ~3-5 years of recent, additional seismic data from the Amundsen Sea and Pine Island Bay regions. Several years of additional data from the Ronne-Fichtner Ice Shelf, Ellsworth Land, and Marie Byrd Land regions will help provide a more complete picture of the mantle transition zone. Data for this study was obtained from IRIS for earthquakes of a 5.5 magnitude or greater. We use an iterative, time domain deconvolution method, filtered with Gaussian widths of 0.5, 0.75, and 1.0. All events within their respective Gaussian filter have undergone quality check by removing waveforms that have lower than 85% fit and visually checking for clear outliers. We migrate the receiver functions to depth and stack, using both single station stacking and common conversion point (CCP) stacking. We migrate the CCP stacks assuming both 1D (AK-135) and 3D velocity models throughout the region. Preliminary results from single-station stacks beneath the Thurston Island and Amundsen Sea regions suggest that the MTZ thickness is similar to the global average and the depth to the transition zone appears to be depressed, with average transition zone boundaries appearing around 430 and 680 km. If the MTZ is thinner than the global average, it may be an indication for high temperature thermal anomalies or a plume under West Antarctica that may help explain the history of extension and uplift there. These results could be useful for glacial isostatic adjustment and/or geothermal heat flux models that attempt to understand ice sheet history and stability.
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Heterogeneous upper mantle structure beneath the Ross Sea Embayment and Marie Byrd Land, West Antarctica, revealed by P-wave tomography
We present an upper mantle P-wave velocity model for the Ross Sea Embayment (RSE) region of West Antarctica, constructed by inverting relative P-wave travel-times from 1881 teleseismic earthquakes recorded by two temporary broadband seismograph deployments on the Ross Ice Shelf, as well as by regional ice- and rock-sited seismic stations surrounding the RSE. Faster upper mantle P-wave velocities (∼ +1%) characterize the eastern part of the RSE, indicating that the lithosphere in this part of the RSE may not have been reheated by mid-to-late Cenozoic rifting that affected other parts of the Late Cretaceous West Antarctic Rift System. Slower upper mantle velocities (∼ −1%) characterize the western part of the RSE over a ∼500 km-wide region, extending from the central RSE to the Transantarctic Mountains (TAM). Within this region, the model shows two areas of even slower velocities (∼ −1.5%) centered beneath Mt. Erebus and Mt. Melbourne along the TAM front. We attribute the broader region of slow velocities mainly to reheating of the lithospheric mantle by Paleogene rifting, while the slower velocities beneath the areas of recent volcanism may reflect a Neogene-present phase of rifting and/or plume activity associated with the formation of the Terror Rift. Beneath the Ford Ranges and King Edward VII Peninsula in western Marie Byrd Land, the P-wave model shows lateral variability in upper mantle velocities of ±0.5% over distances of a few hundred km. The heterogeneity in upper mantle velocities imaged beneath the RSE and western Marie Byrd Land, assuming no significant variation in mantle composition, indicates variations in upper mantle temperatures of at least 100◦C. These temperature variations could lead to differences in surface heat flow of ∼ ±10 mW/m2 and mantle viscosity of 102 Pa s regionally across the study area, possibly influencing the stability of the West Antarctic Ice Sheet by affecting basal ice conditions and glacial isostatic adjustment.
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- Award ID(s):
- 1246151
- NSF-PAR ID:
- 10095101
- Date Published:
- Journal Name:
- Earth and planetary science letters
- ISSN:
- 0012-821X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Brittle faults in the southern Ford Ranges of Marie Byrd Land, West Antarctica, provide unique opportunity to study fluid-rock interactions in the West Antarctic Rift System and the role of crustal fluids during regional-scale faulting. This fault array contains steep, NNW-striking, normal-oblique slip faults and sub-vertical WNW-ESE strike-oblique faults. \ Faults at Mt. Douglass, Mt. Dolber, and Lewissohn Nunatak display strongly aligned tourmaline, indicating syntectonic mineralization; surfaces in one location feature distinctive mirror surfaces, suggestive of formation during seismic slip. Tourmaline has been demonstrated to resist chemical and isotopic re-equilibration during even high-temperature metamorphism, and to maintain a record of conditions during formation, therefore oxygen isotope compositions of tourmaline and quartz pairs may elucidate crustal conditions (e.g. temperatures and fluid-rock ratios) and fluids sources. Analyzed tourmaline and quartz were separated from the upper ~2mm of the fault surfaces; host rocks are tourmaline-free. Tourmaline 18O ratios (n=4) fall within a range of +9.2 to +10.4 ± 0.1 ‰ VSMOW (average 9.7‰, StDev = 0.7). Paired quartz yield 18O values of +11.1 to +10.3 ± 0.1 ‰; ∆Qtz-Trm values between 1.3 and 2.0 may reflect an inability of quartz to equilibrate during tourmaline crystallization. Equilibrium between quartz and tourmaline would suggest temperatures of formation in excess of 550°C. Relative isotopic homogeneity between sites suggests similar fluid conditions were present across the region and supports field evidence for that the structures form a regional fault array. Geometric and kinematic relationships suggest a link to deeper level shears hosting muscovite, and sillimanite with garnet. On-going investigation includes O isotope analyses of these shears, as well as analysis of H and B isotopes in tourmaline, which will better characterize the relationship between the deeper crustal shears with the brittle fault array, and the fluid sources and metasomatic processes of regional fault systems. Furthermore, temporal constraints on tourmaline mineralization will establish whether faulting is associated with Cretaceous intracontinental extension of the West Antarctic rift system (Siddoway 2008) or a crustal response to Neogene mantle delamination beneath the South Pole region (Shen et al 2018).more » « less
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Abstract The upper mantle and transition zone beneath Antarctica and the surrounding oceans are among the poorest‐imaged regions of the Earth's interior. Over the last 15 years, several large broadband regional seismic arrays have been deployed, as have new permanent seismic stations. Using data from 297 Antarctic and 26 additional seismic stations south of ~40°S, we image the seismic structure of the upper mantle and transition zone using adjoint tomography. Over the course of 20 iterations, we utilize phase observations from three‐component seismograms containing
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null (Ed.)Abstract The deployment of seismic stations and the development of ambient noise tomography and new analysis methods provide an opportunity for higher resolution imaging of Antarctica. Here we review recent seismic structure models and describe their implications for the dynamics and history of the Antarctic upper mantle. Results show that most of East Antarctica is underlain by continental lithosphere to depths of ∼ 200 km. The thickest lithosphere is found in a band 500-1000 km west of the Transantarctic Mountains, representing the continuation of cratonic lithosphere with Australian affinity beneath the ice. Dronning Maud Land and the Lambert Graben show much thinner lithosphere, consistent with Phanerozoic lithospheric disruption. The Transantarctic Mountains mark a sharp boundary between cratonic lithosphere and the warmer upper mantle of West Antarctica. In the Southern Transantarctic Mountains, cratonic lithosphere has been replaced by warm asthenosphere, giving rise to Cenozoic volcanism and an elevated mountainous region. The Marie Byrd Land volcanic dome is underlain by slow seismic velocities extending through the transition zone, consistent with a mantle plume. Slow velocity anomalies beneath the coast from the Amundsen Sea Embayment to the Antarctic Peninsula likely result from upwelling of warm asthenosphere during subduction of the Antarctic-Phoenix spreading center.more » « less
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/doi:10.1016/j.epsl.2019.02.013
